The present invention relates to an oxidation-reduction reagent comprising
(a) one or more compounds selected from the group consisting of salts of substituted anthraquinonesulfonic acids of the general formula: ##STR2## wherein M stands for a sodium atom, a potassium atom or an ammonium group and n is an integer of 1 or 2, or (b) a mixture of a salt of anthraquinone-2,7-disulfonic acid and one or more compounds shown by the general formula (I) and/or (II), and also to a process for preparing these compounds.
In the past, it was reported that salts of 2-carboxyanthraquinonemonosulfonic acid shown by the general formula (I) was formed by sulfonation of 2-carboxyanthraquinone with 40% fuming sulfuric acid [A. G. Perkin, J. Chem. Soc. 65, 842 (1894)]. However, this reference nowhere gives a description of the use of this compound and salts thereof. No study has been reported since then on this compound. In recent years, it was disclosed that 2-carboxyanthraquinonedisulfonic acid is formed by heating 2-methylanthraquinone at 140.degree.-170.degree. C. with 30-50% fuming sulfuric acid and that this product can be used in place of anthraquinonedisulfonic acid in a wet desulfurization process according to the Stretford process (British Pat. No. 1,508,268).
Sulfonated products of 2-phenoxyanthraquinone which are shown by the general formula (II) are new compounds and have not been known hitherto.
In a wet desulfurization process, salts of anthraquinonedisulfonic acid are well known as useful oxidation-reduction reagents.
By the term "wet desulfurization process" is meant a process wherein a petroleum gas containing hydrogen sulfide, coal gas or a liquid hydrocarbon is brought into contact with a liquid containing an oxidation-reduction reagent capable of being reversibly oxidized or reduced, whereby the hydrogen sulfide is converted by oxidation into elementary sulfur and eliminated and, on the other hand, the oxidation-reduction reagent reduced by the oxidation of hydrogen sulfide is oxidized by contacting the reduced reagnet with air for regeneration, and this cycle is repeated for oxidation of hydrogen sulfide.
Among the wet desulfurization processes of the above mentioned type, one especially called "Stretford process" employs as an oxidation-reduction reagent a basic aqueous solution of (a) one or more anthraquinonedisulfonic acid or salts thereof or (b) a mixture of these with a compound of a metal capable of possessing two different kinds of valence, preferably a vanadate such as sodium metavanadate, optionally with a chelating agent such as potassium sodium tartarate or EDTA (Japanese Pat. Nos. 401,331, 405,170 and 405,171). In one of the most practical embodiments of this Stretford process where a basic aqueous solution of anthraquinonedisulfonates and sodium metavanadate is used as an oxidation-reduction reagent, the following reaction is surmised to take place ["Aromatics" 19, 53 (1967) published by Shadanhojin Nichon Hokozoku Kogyokai (Japan Aromatic Industry, Inc.)]: EQU 2Na.sub.2 CO.sub.3 +2H.sub.2 S.fwdarw.2NaHS+2NaHCO.sub.3 EQU 4NaVO.sub.3 +2NaHS+H.sub.2 O.fwdarw.Na.sub.2 V.sub.4 O.sub.9 +2S+4NaOH EQU Na.sub.2 V.sub.4 O.sub.9 +2NaOH+2ADA.fwdarw.4NaVO.sub.3 +2ADA* EQU 2ADA*+O.sub.2 (air).fwdarw.2ADA+2H.sub.2 O EQU 2NaHCO.sub.3 +2NaOH.fwdarw.2Na.sub.2 CO.sub.3 +2H.sub.2 O
In the above formulas, ADA stands for a salt of anthraquinonedisulfonic acid and ADA* for a leuco form of the salt of anthraquinonedisulfonic acid.
As is evident from the above reaction formulas, a salt of anthraquinonedisulfonic acid used in the Stretford process participates together with sodium metavanadate in the oxidation of hydrogen sulfide to elementary sulfur whereby the salt itself is reduced to a leuco form which is then oxidized to regenerate the salt when brought into contact with air. Heretofore, salts of anthraquinone-1,6-, -1,7- and -2,7-disulfonic acids are known to be effective as a salt of anthraquinonedisulfonic acid for the Stretford process. As a high water-solubility and a commercial availability at a low cost are required for practical use, however, salts of anthraquinonedisulfonic acids now practically utilizable are limited only to a salt of anthraquinone-2,7-disulfonic acid. Thus, the following serious problems arise in such situation.
Disulfonation of anthraquinone for preparing a salt of anthraquinone-2,7-disulfonic acid affords a mixture of almost equiamounts of anthraquinone-2,6- and -2,7-disulfonic acids. A salts of anthraquinone-2,6-disulfonic acid is sparingly soluble in a basic aqueous solution used in the Stretford process and is thus hardly utilizable for this process. If the salt of anthraquinone-2,6-disulfonic acid finds no useful application, it will become extremely difficult to supply a salt of anthraquinone-2,7-disulfonic acid at a low cost for the Stretford process. In addition, the use of anthraquinone-2,6-disulfonic acid is limited to the preparation of specific dye intermediates, whereas the Stretford process distinguished by its extremely high desulfurization ratio permitting the effect that the content of the remaining hydrogen sulfide in the refined gas treated according to this process is extremely low is now being adopted gradually widely. Thus, a significant inbalence in utility is found between the 2,6-disulfonic acid and the 2,7-disulfonic acid, and there is a great demand for a reagent capable of substituting itself for the 2,7-disulfonic acid without such limitation.
To fill the demand, 2-carboxyanthraquinonedisulfonic acids were proposed which are obtained by heating 2-methylanthraquinone with fuming sulfuric acid at a high temperature (DOS. No. 2,639,365 and British Pat. No. 1,508,268). According to the present inventors' experiments, however, it has been found that 2-carboxyanthraquinonedisulfonic acids obtained in the above mentioned manner are in the form of a complex mixture comprising at least 7 isomers and decomposition products having indefinite structures and that as will be described hereinafter, a difficult problem arises in the formation of by-products which are inferior in solubility in a concentrated inorganic brine containing a base, the solubility being one of the properties required for oxidation-reduction reagents for use in a wet desulfurization process.
It is necessary that oxidation-reduction reagents used for a wet desulfurization process, especially the Stretford process be furnished with such properties and economical merits as will be itemized below.
(1) Good solubility in an absorbing liquid used in the wet desulfurization process (In the wet desulfurization process, a weakly alkaline absorbing liquid is circulated in the system for the treatment of a gas containing hydrogen sulfide whereby the hydrogen sulfide is not oxidized to elemental sulfur at a conversion rate of 100% and a part of the hydrogen sulfide is oxidized to thiosulfates and sulfates or converted into rhodanides when cyanides are present in the feed gas. Since these by-products are built up in the absorbing liquid, the concentration of inorganic salts in the absorbing liquid will reach a level as high as 300-400 g/l after a continuous run for a long period of time. Thus, a practically utilizable oxidation-reduction reagent should possess a good solubility even in a highly concentrated solution of inorganic salts.)
(2) Moderate oxidation-reduction capacity and rate (As is evident from the reaction formulas previously given, a combination of sodium vanadate and anthraquinonesulfonates constructs an oxidation-reduction system. Consequently, the anthraquinonesulfonates used as catalyst are required to have a moderate oxidation-reduction capacity and rate).
(3) Minimum consumption (As the oxidation-reduction cycle is repeated in the absorbing liquid, it is unavoidable that the catalyst is gradually consumed by side-reactions. However, the consumption of the catalyst should be as minimum as possible.)
(4) Low toxicity and easiness in the treatment of waste liquor
(5) In the preparation of the oxidation-reduction reagents, the formation of no by-product which is sparingly soluble and cannot be used for the Stretford process
(6) Easiness in substitution for anthraquinone-2,7-disulfonate used hitherto for the Stretford process
(7) Possibility of the conjoint use with anthraquinone-2,7-disulfonate for facilitating the substitution work stated in item (6)
(8) Possibility of producing the end product of high purity as simple in the steps as possible.
As a result of our researches made for synthesis of a great number of anthraquinonesulfonates with a view to developing an oxidation-reduction reagent which can fully satisfy the foregoing requirements and for testing of the catalytic performance of these anthraquinonesulfonates in the Stretford process, it has now been found that the compounds of the above general formulas (I) and (II) fully satisfy the foregoing requirements and exhibit an excellent catalytic performance. The present invention has been accomplished on the basis of the above finding.